Heterogeneity of Framework Ti(IV) in Ti−Silicalite as Revealed by the Adsorption of NH3. Combined Calorimetric and Spectroscopic Study

The room-temperature adsorption of NH{sub 3} on Ti-silicalite (TS-1) was studied by means of IR and XANES-EXAFS spectroscopies and by microcalorimetry. The propensity of framework tetrahedral Ti(IV) heteroatoms to expand their coordination shell upon adsorption of NH{sub 3} was monitored by the perturbation induced on the Ti-sensitive framework stretching band (960 cm{sup {minus}1}) and on the preedge peak at 4967 eV in XANES spectra. In both cases a modification of the local geometry of titanium sites upon adsorption of NH{sub 3} as an additional ligand was deduced. The amount of NH{sub 3} specifically adsorbed on the Ti(IV) sites was estimated by comparing volumetric data obtained for TS-1 with those obtained for a Ti-free silicalite taken as reference material. At p{sub NH{sub 3}} = 50 Torr, the number of NH{sub 3} molecules adsorbed per Ti atom was found to be close to 2, indicating that virtually all Ti atoms are involved in the interaction and have completed their 6-fold coordination shell. The molar heats of adsorption on the Ti(IV) sites were estimated following the same procedure. It was found that the molar heat of NH{sub 3} adsorption on titanium is higher (q{sub m} = 95--83 kJ/mol) than that measured for themore » silicalite matrix (q{sub m} = 66--58 kJ/mol). The interaction with ammonia was found to be essentially reversible at room temperature but for a significant amount of adsorbed species irreversibly held on titanium sites. The zero-coverage heat of adsorption was quite high (q {approx} 200 kJ/mol) not only for the first run of adsorption involving both irreversible and reversible interaction but also for the second run involving only the reversible component (q {approx} 130 kJ/mol). The evolution of the heat of adsorption with coverage was typical of heterogeneous surfaces; this is due not only to the presence of sites active toward ammonia on the silica matrix but also to the heterogeneous distribution of Ti(IV) sites, suggesting that a considerable number of framework sites (among the 12 available in the orthorhombic MFI framework) are occupied in a nearly equidistributed way. EXAFS measurements allowed the elongation of the Ti-O bond upon interaction with NH{sub 3} (0.05 {+-} 0.03 {angstrom}), the Ti-N distance (1.93 {+-} 0.03 {angstrom}), and the average number of adsorbed NH{sub 3} molecules per Ti site (1.9 {+-} 0.3) to be estimated. The results were in good agreement with the volumetric-calorimetric data. The effect of the pretreatment of TS-1 samples with ammonium acetate aqueous solution was also investigated by comparing the spectroscopic and the quantitative calorimetric results for both untreated and treated TS-1 samples.« less